The North American Radio Broadcasting Agreement

An added bit of confusion for those interested in the early history
of radio in North America is a treaty called the North American Radio
Broadcasting Agreement, usually referred to as ``NARBA'', which
rearranged the radio dial substantially. The rationale behind the
treaty was to set aside Canadian (and Mexican) clear channel
frequencies throughout the dial, at a rate of about one per 100 kHz.
The treaty also expanded the band from 1500 kHz to 1600 kHz; it took
effect on March 29, 1941. This agreement is no longer in force; it
has been superseded by US-Canada and US-Mexico working arrangements
made in the early 1980s, as well as an ITU-sponsored agreement
covering all of the Western Hemisphere.